1. Field of the Invention
This invention relates to synthetic resin coatings capable of dissipating surface electrostatic charge and more particularly to siloxane coatings having abrasion resistance and permanent static-dissipation properties.
2. Brief Description of the Prior Art
Polysiloxane coatings have been extensively used as protective coatings for articles made from synthetic resins in order to provide the relatively soft resin with a hard, abrasion-resistant surface. Curable coating compositions for applying such abrasion-resistant coatings are disclosed, for example, in U.S. Pat. Nos. 3,986,997 and 4,246,038. The entire disclosure of U.S. Pat. Nos. 3,986,997 and 4,246,038 is incorporated herein by reference. Such abrasion-resistant coatings are desirable and even necessary in applications such as coating of optically transparent materials such as plastic window glazing, protective screens for cathode ray tubes and screens for rear-projection television apparatus. For such articles, surface scratches are especially undesirable because the accumulation of such scratches causes the article to become hazy and thereby decreases the transparency of the window, viewing screen, or the like.
However, hard, abrasion-resistant coatings tend to have a very high surface resistivity, which makes them prone to accumulate static charge and very slow to dissipate that charge. The static charge on the surface tends to attract dust, which also causes a loss of clarity of the transparent article. In some cases the static charge may even interfere with the operation of electrical equipment positioned close to the charged surface, e.g., within a plastic cabinet that accumulates static surface charge.
Auxiliary anti-static coatings, sprayed onto the surface, have been used to alleviate the problem of retained static charge. However, such sprayed coatings are not permanent, but must be renewed from time to time.
Accordingly, a need has continued to exist for a hard, abrasion-resistant coating that has satisfactory permanent static electricity dissipation properties, especially for use in coating transparent substrates.